1. Field of the Invention
This invention relates to a UHF (Ultra High Frequency) electronic tuner suitable for use in a television receiver, and more particularly to a UHF electronic tuner in which a lumped constant tuning circuit is used together with a distributed constant tuning circuit so as to take advantage of the merits of both types of circuits and to avoid their disadvantages.
2. Description of the Prior Art
Generally, a UHF electronic tuner includes an RF (Radio Frequency) input circuit, an RF amplifying circuit, a mixing circuit and a local osillating circuit which are coupled to each other through respective tuning circuits. Circuits for UHF electronic tuners are roughly classified into lumped constant tuning circuit types utilizing lumped constant elements such as capacitors, inductors and resistors and distributed constant tuning circuit types utilizing distributed constant elements such as resonant cavities.
Lumped constant circuit elements are normally mounted on printed-circuit boards and thus are easily mass produced using conventional automatic equipment. However, when used in UHF tuning circuits, lumped constant tuning circuits have so much loss, particularly when used in an RF input circuit, that deterioration in the noise figure (NF) may occur. Moreover, lumped constant circuits have the additional disadvantage that they do not offer an easy way to convert the normal balanced antenna input impedance of between about 200 and 300 ohms to the required unbalanced input impedance of about 75 ohms.
Distributed constant circuits are divided into a first type characterized by strip-line circuits and a second type characterized by semicoaxial resonant cavity circuits consisting of an outer conductor which forms a cavity and an inner conductor placed within that cavity. Strip-line circuits, which include one type which can be formed on a printed-circuit board, are more easily mass produced but they have the disadvantage that they also do not offer an easy way to convert a balanced antenna input impedance of between 200 and 300 ohms to the required unbalanced input impedance of about 75 ohms. In a distributed constant device of the semicoaxial resonant cavity type, a metal case is partitioned into a plurality of compartments by partitioning plates. The necessary circuits are placed in the respective compartments, and are coupled to each other using tuning circuits employing an inner conductor and a cavity. This type is widely used, and has the desirable characteristics of low loss and ease of accommodating either unbalanced or balanced antenna inputs. However, semicoaxial devices have the disadvantage that they are more difficult and costly to mass produce since the inner conductor must be accurately spatially arranged with respect to an input coupling device and to the cavity, and they are thus less adapted to mass production and are more costly.
In lumped constant or strip-line circuits, a .lambda./2-balun utilizing a strip line element is often used as an impedance converter from 300 ohms balanced to 75 ohms unbalanced in order to match the antenna impedance to the required input impedance. Such a balum does not maintain a good impedance match over the wide frequency range of the UHF band. Interference from HF (High Frequency) or VHF (Very High Frequency) signals can therefore be picked up by feeder extending from an antenna to the receiver and interfere with the desired UHF signal. Accordingly, a high-pass filter is also normally required in the input circuit to reject such interference.